Resistance unit



Nov. 29, 1932. o, WHELAN 1,889,618

RESISTANCE UNIT Filed March 8, 1929 Inventor: Dow O. Wlqelan by His Attorney.

Patented Nov. 29, I932 Umrso STA 'rss PATENT OFFICE .DOW O. WHELAN, F SCOTIA, NEW YORK, ASSIGNORIG GENERAL ELECTRIC COMPANY,

' A. CORPORATION OF NEW YORK nnsisranon UNIT Application filed March 8,

My invention relates to resistance units and the like, and has for its object the pro-vision of a simple and eflicient method and means for making a resistance unit whereby a sturdy, compact and durable resistance unit is pro-' vided. V

Although it obviously has other applications, my invention is particularly useful in connection with resistance units of relatively high resistance and low current-carrying capacity for use in radio and like circuits.

Certain resistance units having a relatively high resistance and low current-carrying capacity, hereto-fore, have been provided, Grenerally these units comprised a bar or strip formed of or coated with some high resistance material as, for instance, carbon or graphite and a number of terminal members spaced along the bar. In many instances the terminals were secured by being bent par tially about the bar so as to assume a substantially hairpin shape. Ihe legs of these terminals were secured to the bar by strips of insulating fabric which in turn were bound in position by a moulded insulating compound as, for instance, bakelite. Resistance units of this character were more or less objection able because the insulating fabrics often times absorbed moisture from the atmosphere in 39 such quanities that they became warped and distorted. 'By reason of this, the terminals were no longer securely held in firm contact with the carbon bar and as a result the resistance between the terminalsvaried considerably at diii'erent times.

In carrying my invention into effect in one form thereof, I embed the terminal members in a suitable resistance medium so that they will be rigidly supported by the resistance medium itself. Preferably, I employ a moulded carbon compound as the resistance medium and embed the terminal members. during the moulding operation.

For a more complete understanding of my invention, reference should be had to the ac companying drawing in whichFig. 1 illus-. trates a step in the method of-forming a resistance unit in accordance with my inveni tion; Fig. 2 illustrates the resistance unit in v 1929. Serial No. 345,457..

finished form; and Fig. 3 is a bottom plan of the finished resistance unit.

Referring to the drawing, I have show my invention in one form in connection with a resistance unit particularly useful as a rheostat or potentiometer in radio and like circuits. As constructed in accordance with my invention, the resistance unit in finished form comprises a substantially cylindrical moulded carbon resi tance body portion 10, a plurality of longitudinally spaced contact terminals 11 and a pair of end terminals 12, one of which will be provided for each end ofthe resistance unit. Each terminal will have a portion embedded in the moulded carbon body so as to be rigidly supported thereby in good electrical contact with the resistance material. .lt will be understood, of course, that the moulded. body may assume other shapes suitable to desired installations or to meet the requirements of a particular use.

In accordance with my invention, the resistance body is moulded or otherwise formed as a unit in any desired shape, In making the resistance unit I position a suitable electrical conductor 13, formed substantially as a helix (Fig. l), in the mould in which the carbon portion; body 10 is to be formed. It will be understood that the longitudinal axis of the helix will be shaped in conformity with the longitudinal axis of the form to be assumed by the moulded body portion and preferably will coincide with this axis. Al-

' though any suitable electrical conductor may any other suitable arrangement of the conductor ends may be used.

After the helix has been positioned in the mould, carbon, preferably in the form of flakes, together with some suitable binding material will 'be packed into the mould in and about all of the turns of the helix, it.

being understood that the mould will be of such proportions that the finished unit will have a diameter slightly larger than that of the helix. Moreover, the finished unit preferably will be of such length that the inturned ends of the helix, i. e., the end terminals 12 will be exposed. Thus as shown, the inner surfaces of the terminals 12 will be substantially flush with the end surfaces of the unit. It will be understood, however, that this is not a necessary arrangement since the ends of the conductor may be embedded in the moulded unit. Although any suitable binding material may be used, I prefer to use a hydro-carbon binder such as coal tar, coal tar pitch, etc. It will be understood that the binder will be of sufiicient content to give the desired mechanical strength. Thus, the binder should be added in such proportions that substantially all the carbon flakes will be coated with the binding material.

In. the next step the unit is heated under pressure so as to drive off the volatile constituents of the binding material and to compact the structure of the unit. Preferably, the unit will be heated to a temperature high enough to cause practically complete carbonization of the binding material. It will be understood, however, that the temperature to which the unit is heated and the duration of the heating period will be determined by the resistance required, the size of the unit, the proportion of binding material used and the conducting material from which the helix is formed. Likewise, the pressure to be applied will vary with the particular unit being treated. Preferably, however, the pressure will be high enough to press the carbon to a compact, homogeneous mass.

After. the heat and pressure treatment, the unit will be removed from the mould and the carbon on one side of the unit will be removed in any suitable manner so as to expose a portion of each conductor turn, as shown in Figs. 2 and 3. Preferably, a longitudinal strip 14 of the carbon throughout the entire length suitable rotary wire brush. It will be understood that the exposed portion of each turn will serve as a contact terminal member for which any suitable contact arm (not shown) or soldered connection may be provided.

In order to prevent the current fromtraveling along the helix in the operation of the unit, the turns of the helix will be separated from each other by the removal of a portion of each turn. This may be effected conveniently by milling a, longitudinal slot 15 of a depth suflicient to sever the conductor turns. It will be observed that after the slot 15 has been milled, each turn will be shaped substantially as a broken ring, practically the entire length of which will be embedded in the carbon body.

It has been found that certain resistance units, particularly those formed from a carbon compound, will deteriorate when subjected to the vapors from certain oils, beeswax, certain paraflin compounds, etc. To prevent this, I coat the exterior portions of the unit, other than the terminals, with a suitable protective lacquer or varnish. Preferably, this lacquer will be of a pyroxylin base. In coating the unit, it is but necessary to dip the unit in the lacquer and then .withdraw it to dry. Of course, the terminals will be wiped or brushed free of the lacquer.

It will be observed that the resistance unit in finished form comprises a resistance body portion having contact terminals spaced longitudinally thereof and a terminal in each end all of which terminals will be securely anchored to the resistance body. Thus,'the resistance unit may conveniently be used as a potentiometer in which the two end terminals will be employed together with the spaced terminals or it may be used as a rheostat in which all or any desired number of the terminals may be employed. It will be understood that the resistance between each terminal may be determined by the spacing of the turns of the helix and by the amount of carbon brushed away in the operation of exposing the turns. Thus, if the turns are spaced closer together, the resistance between turns will be lessened; likewise, if more of the carbon is brushed away this resistance will be increased. 4

It is to be noted that the heat and pressure treatment will effect a very hard, compact and mechanically strong resistance unit.

The unit will not deteriorate under the influence of atmospheric moisture, vapors of wax, certain oils, paraflins, etc., nor will the terminals be loosened either under the action of atmospheric moisture or under the action of ordinary me'chanicalshocks or strains.

Moreover, it is to be noted that the helix may be positioned eccentrically within the mould so that after the carbon has been moulded a portion of each turn will be exposed without the brushing operation. Of course, the turns will be separated by the millingorsome other like operation in finishing the unit. I

WVhile I have in accordance with the provisions of the patent statutesdescribed my invention as embodied in concrete form, it should be understood that I do not limit myinvention the cto, since various modifications thereof will suggest themselves to those skilled in the art without departing from the spirit of my invention, the scope of which is set forth in the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A resistance unit or the like comprising a helical electrical conductor and a body of electrical resistance material embedding said conductor so as to leave a portion of each of a plurality of the convolutions of said conductor exposed for connection with an electric circuit, said body being provided with a longitudinally extending slot intersecting said convolutions, each of said convolutions terminating with the side walls of said slot.

2. The method of making a resistance unit or the like which comprises completely embedding a substantially helical electrical conductor in a moulded resistance medium so as to forma bar with said helical conductor embedded therein arranged lengthwise of the bar cutting through said medium lengthwise of said bar so as tosever each turn of said conductor and then cutting away said resistance medium to expose a portion of each turn so that each turn functions as a terminal member.

3. The method of making a resistance unit or the like which consists in providing a substantially helical electrical conductor, completely embedding said helical conductor in a mixture of carbon and h drocarbon binding material, moulding said elical conductor in said mixture of carbon and hydrocarbon binding material, heating to drive off the volatile constituents of said hydro-can bon binding material While applying pressure to compact the carbon into a homogeneous mass with said conductor embedded therein forming a slot in said mass so as to sever each turn of said conductor and then cutting away the resistance medium to expose a portion of each turn whereby each turn functions as a terminal member.

4. The method of making a resistance unit or the like which consists in providing a substantially helical electrical conductor, embedding said helical conductor in a bar of carbon composition so that the ends of said conductor project from the ends of said bar to rovide end. terminal members therefor, mil ing a slot lengthwise of said bar so as to sever each turn of said helical conductor and then brushing the carbon composition.

free from a portion of each turn ofsaid conductor so as to provide terminal members spaced longitudinally of said bar.

5. The method of making a resistance unit or the like which comprises completely embedding a substantially helical electrical conductor in a mouldable resistance medium; moulding said medium to form a bar of resistance material with said helical conductor other than the exposed portions, with a pro tective varnish.

In witness whereof, I have hereunto set my hand this 7th day of March, 1929.

DOW O. WHELAN. 

